This invention relates to the petrophysical evaluation of subsurface formations and more particularly to a subsurface formation petrophysical evaluation system that is capable of providing reliable and accurate petrophysical answers, such as porosity, permeability, and hydrocarbon/water saturation, soon after data acquisition, such as at the wellsite.
There is a long history of wanting to provide petrophysical answers (comprising at least porosity and hydrocarbon/water saturation) in xe2x80x9creal timexe2x80x9d, which in the context of this application means soon after data collection, preferably at the wellsite. Past attempts to provide such a product have been plagued by the need to define numerous parameters and well zonations that would ordinarily need to be selected by a skilled interpreter. Due to the extensive time, effort, and expertise required, computed petrophysical evaluations are not currently performed for many wells. Reliable and accurate estimates of formation parameters could act as valuable starting points for wells where mineral model inversion will ultimately be used to compute reserves. Rapid petrophysical evaluation could also help oil and gas companies more quickly prioritize the wells and formations within those wells that appear to offer the best opportunities for improving return on investment. A real time petrophysical evaluation system implemented in a while drilling environment would also allow drilling parameters to be determined, such as changes to drilling direction and when to stop drilling.
For these reasons, it would be of great benefit to be able to provide a subsurface formation petrophysical evaluation system capable of providing reliable and accurate petrophysical answers, such as porosity, permeability, and hydrocarbon/water saturation, in real time, such as at the wellsite.
One aspect of the invention involves a subsurface formation petrophysical evaluation method including: determining formation porosity and permeability using a quantitative indication of formation composition; estimating water-filled formation resistivity using the formation porosity; associating differences between measured formation resistivity and estimated water-filled formation resistivity with the presence of subsurface hydrocarbons; and estimating irreducible formation water saturation (also called capillary bound water) using the formation porosity and the formation permeability. An apparatus adapted to carry out the inventive method is also described. Other aspects of the invention involve making a plurality of different measurements of a subsurface formation, including indicators of a plurality of absolute or relative formation elemental concentrations; processing the measurements to determine a plurality of parameters associated with the subsurface formation, and determining either net pay intervals or drilling parameters using these formation parameters. Further details and features of the invention will become more readily apparent from the detailed description that follows.